Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$35.27	13.353%	$39.9796	$39.98	$40.00	$40.00
Purchase #2	$218.61	13.135%	$247.3244	$247.32	$247.30	$247.30
Purchase #3	$88.65	12.816%	$100.0114	$100.01	$100.00	$100.00
Purchase #4	$65.37	9.408%	$71.52	$71.52	$71.50	$71.50
Purchase #5	$147.95	9.458%	$161.9431	$161.94	$161.95	$161.90
Purchase #6	$103.83	5.165%	$109.1928	$109.19	$109.20	$109.20
Purchase #7	$38.30	12.856%	$43.2238	$43.22	$43.20	$43.20
7 purchase totals =	$697.98		$773.1951	$773.18	$773.15	$773.10